Assessment of mitigation measures to reduce interactions between sea turtles and longline fishery

Sea turtles have been receiving negative impacts from both human activities and natural factors. Interactions with commercial fisheries are one of the anthropogenic factors affecting sea turtle populations. A variety of mitigation measures have been developed and tested to reduce incidental mortality of sea turtles in longline fishery. We review potential technical measures to alleviate longline-sea turtle interactions and show some preliminary results from our field and laboratory experiments. Fishing gear modifications aim at reducing hooking rates of sea turtles by changing fishing hooks and baits or by the use of additional devices. Results of our field experiments showed that the use of circle hooks altered hooking position and reduced deep hooking of sea turtles. Fish baits showed lower catch rates of sea turtles than squid baits, because sea turtles were more likely to swallow the whole squid bait due to tough and flexible muscle texture of squids. Fishing practice modifications aim at avoiding the overlap between fishing operations and sea turtles either spatially or temporally. In the oceanic area, sea turtles spend most of their time within the shallow surface layer (<40m). Deep-setting longline is effective to avoid incidental capture of sea turtles. Sea turtles have habitat preference for warm water, and migratory species seem to have distinct routes for long-distance migration. Biotelemetry studies play a important role because they provide baseline information on habitat utilization, diving profile, activity pattern, and migratory paths of sea turtles. Careful handling and live release is another way to reduce post-hooking mortality of sea turtles because many sea turtles captured in shallow longline are retrieved alive. Several instruments have been developed to haul sea turtles onboard and to remove fishing hooks and lines. Results of our captive experiments indicated that hooked sea turtles survived for a prolonged period and discharged fishing hooks out of the body. Since these mitigation techniques affect the fishing efficiency of target species, we should also assess the economic feasibility of each method to establish a practical solution.